1. Field of the Invention
The present invention relates to an anode having an anode active material layer on an anode current collector and a battery including the anode.
2. Description of the Related Art
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery can provide a higher energy density compared to a lead battery and a nickel cadmium battery. The lithium ion secondary battery has a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer on an anode current collector.
As an anode active material contained in the anode active material layer, a carbon material such as graphite has been widely used. In recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon, tin or the like instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) and the theoretical capacity of tin (994 mAh/g) are significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
However, in the case where silicon or the like is used as the anode active material, the anode active material inserting lithium when charged is highly activated. Thus, the electrolytic solution is easily decomposed, and lithium is easily inactivated. Thereby, the discharge capacity is lowered when charge and discharge are repeated, and thus sufficient cycle characteristics are hardly obtained.
Therefore, in the case where silicon or the like is used as the anode active material, various devices have been invented as well to improve the cycle characteristics. Specifically, a technique that perfluoro polyether is contained in the electrolytic solution (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2002-305023 and 2006-269374), and a technique that a coat containing perfluoro polyether is provided on the surface of an anode (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-265609) have been proposed. In addition, a technique that an anode is covered with a polymer material such as a homopolymer or a copolymer of vinylidene fluoride (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-517719) has been proposed.